**Environment**: server.

To track bugs, we use `onBug()`:

```ts
import { onBug } from 'telefunc'

onBug((err) => {
  // ...
})
```

This allows us, for example, to install the tracker code of some tracking service ([Sentry](https://sentry.io/), [Bugsnag](https://www.bugsnag.com/), [Rollbar](https://rollbar.com/), ...).

`onBug()` is called:
 - When a telefunction throws an error that is not `Abort()`. (In other words, our telefunction has a bug.)
 - When Telefunc throws an error. (In other words, Telefunc has a bug).

> `throw Abort()` does *not* trigger `onBug()`. It's expected that `throw Abort()` may be called (when some third-party erroneously calls a telefunction). In other words, `throw Abort()` isn't a bug.
